Method and system for adaptive allocation of feedback resources for CQI and transmit pre-coding

ABSTRACT

Aspects of a method and system for adaptive allocation of feedback resources for CQI and transmit pre-coding may include assigning a bandwidth and a feedback period to one or more CQI reporting units and a bandwidth and a feedback period to one or more PMI reporting units. One or more feedback messages may be generated based at least on channel state information associated with the bandwidth and the feedback period assigned to the one or more CQI reporting units and with the bandwidth and the feedback period assigned to the one or more PMI reporting units. The bandwidth and the feedback period of the CQI reporting units and/or the PMI reporting units may be adjusted dynamically and/or adaptively. Based on the channel state information or as a function of a feedback rate, the bandwidth and/or the feedback period may be assigned.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to, claims priority to, and claims thebenefit of U.S. Provisional Application Ser. No. 60/896,121, filed onMar. 21, 2007.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing forcommunication systems. More specifically, certain embodiments of theinvention relate to a method and system for adaptive allocation offeedback resources for CQI and transmit pre-coding.

BACKGROUND OF THE INVENTION

Mobile communications have changed the way people communicate and mobilephones have been transformed from a luxury item to an essential part ofevery day life. The use of mobile phones is today dictated by socialsituations, rather than hampered by location or technology. While voiceconnections fulfill the basic need to communicate, and mobile voiceconnections continue to filter even further into the fabric of every daylife, the mobile Internet is the next step in the mobile communicationrevolution. The mobile Internet is poised to become a common source ofeveryday information, and easy, versatile mobile access to this datawill be taken for granted.

Third generation (3G) cellular networks have been specifically designedto fulfill these future demands of the mobile Internet. As theseservices grow in popularity and usage, factors such as cost efficientoptimization of network capacity and quality of service (QoS) willbecome even more essential to cellular operators than it is today. Thesefactors may be achieved with careful network planning and operation,improvements in transmission methods, and advances in receivertechniques. To this end, carriers need technologies that will allow themto increase downlink throughput and, in turn, offer advanced QoScapabilities and speeds that rival those delivered by cable modem and/orDSL service providers.

In order to meet these demands, communication systems using multipleantennas at both the transmitter and the receiver have recently receivedincreased attention due to their promise of providing significantcapacity increase in a wireless fading environment. These multi-antennaconfigurations, also known as smart antenna techniques, may be utilizedto mitigate the negative effects of multipath and/or signal interferenceon signal reception. It is anticipated that smart antenna techniques maybe increasingly utilized both in connection with the deployment of basestation infrastructure and mobile subscriber units in cellular systemsto address the increasing capacity demands being placed on thosesystems. These demands arise, in part, from a shift underway fromcurrent voice-based services to next-generation wireless multimediaservices that provide voice, video, and data communication.

The utilization of multiple transmit and/or receive antennas is designedto introduce a diversity gain and to raise the degrees of freedom tosuppress interference generated within the signal reception process.Diversity gains improve system performance by increasing receivedsignal-to-noise ratio and stabilizing the transmission link. On theother hand, more degrees of freedom allow multiple simultaneoustransmissions by providing more robustness against signal interference,and/or by permitting greater frequency reuse for higher capacity. Incommunication systems that incorporate multi-antenna receivers, a set ofM receive antennas may be utilized to null the effect of (M−1)interferers, for example. Accordingly, N signals may be simultaneouslytransmitted in the same bandwidth using N transmit antennas, with thetransmitted signal then being separated into N respective signals by wayof a set of N antennas deployed at the receiver. Systems that utilizemultiple transmit and receive antennas may be referred to asmultiple-input multiple-output (MIMO) systems. One attractive aspect ofmulti-antenna systems, in particular MIMO systems, is the significantincrease in system capacity that may be achieved by utilizing thesetransmission configurations. For a fixed overall transmitted power andbandwidth, the capacity offered by a MIMO configuration may scale withthe increased signal-to-noise ratio (SNR). For example, in the case offading multipath channels, a MIMO configuration may increase systemcapacity by nearly M additional bits/cycle for each 3-dB increase inSNR.

The widespread deployment of multi-antenna systems in wirelesscommunications has been limited by the increased cost that results fromincreased size, complexity, and power consumption. As a result, somework on multiple antenna systems may be focused on systems that supportsingle user point-to-point links, other work may focus on multiuserscenarios. Communication systems that employ multiple antennas maygreatly improve the system capacity. To obtain significant performancegains using MIMO technology, it may however be desirable to supplyinformation on the channel to the transmitter. Such channel data iscalled channel state information (CSI). In many wireless systems, theuplink and the downlink operate in frequency division duplex (FDD) mode,that is, the uplink and the downlink use different frequencies. Whenthis is the case, channel measurements of the uplink may not beapplicable to the downlink and vice versa. In these instances, thechannel may be measured only by a signal receiver and channel stateinformation may be fed back to the transmitter.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A method and/or system for adaptive allocation of feedback resources forCQI and transmit pre-coding, substantially as shown in and/or describedin connection with at least one of the figures, as set forth morecompletely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram illustrating an exemplary MIMO transceiverchain model, in accordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary MIMO with finite rate channelstate information feedback, in accordance with an embodiment of theinvention.

FIG. 4 is a time-frequency diagram illustrating a time-frequencywireless channel, in accordance with an embodiment of the invention.

FIG. 5 is a flow chart illustrating an exemplary adjustment of CQI andPMI reporting units, in accordance with an embodiment of the invention

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor adaptive allocation of feedback resources for CQI and transmitpre-coding. Aspects of the method and system for adaptive allocation offeedback resources for CQI and transmit pre-coding may compriseassigning a bandwidth and a feedback period to one or more CQI reportingunits and a bandwidth and a feedback period to one or more PMI reportingunits. One or more feedback messages may be generated based at least onchannel state information associated with the bandwidth and the feedbackperiod assigned to the one or more CQI reporting units and with thebandwidth and the feedback period assigned to the one or more PMIreporting units.

The bandwidth and the feedback period of the CQI reporting units and/orthe PMI reporting units may be adjusted dynamically and/or adaptively.Based on the channel state information or as a function of a feedbackrate, the bandwidth and/or the feedback period may be assigned. Theassociated channel state information may comprise channel measurementsthat may be associated with the assigned bandwidth and the assignedfeedback period. Processing the channel state information may beachieved by averaging the channel measurements or by an arbitraryfunction of the channel measurements. The one or more feedback messagesfor the one or more CQI reporting units may comprise modulation andcoding levels or Signal-Noise-and-Interference ratios. The one or morefeedback messages for the one or more PMI reporting units may compriseone or more indices into a codebook or one or more matrices.

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention. Referring to FIG. 1A,there is shown a house 120, a mobile terminal 122, a factory 124, a basestation 126, a car 128, and communication paths 130, 132 and 134.

The base station 126 and the mobile terminal 122 may comprise suitablelogic, circuitry and/or code that may be enabled to generate and processMIMO communication signals.

Wireless communications between the base station 126 and the mobileterminal 122 may take place over a wireless channel. The wirelesschannel may comprise a plurality of communication paths, for example,the communication paths 130, 132 and 134. The wireless channel maychange dynamically as the mobile terminal 122 and/or the car 128 moves.In some cases, the mobile terminal 122 may be in line-of-sight (LOS) ofthe base station 126. In other instances, there may not be a directline-of-sight between the mobile terminal 122 and the base station 126and the radio signals may travel as reflected communication pathsbetween the communicating entities, as illustrated by the exemplarycommunication paths 130, 132 and 134. The radio signals may be reflectedby man-made structures like the house 120, the factory 124 or the car128, or by natural obstacles like hills. Such a system may be referredto as a non-line-of-sight (NLOS) communications system.

A communication system may comprise both LOS and NLOS signal components.If a LOS signal component is present, it may be much stronger than NLOSsignal components. In some communication systems, the NLOS signalcomponents may create interference and reduce the receiver performance.This may be referred to as multipath interference. The communicationpaths 130, 132 and 134, for example, may arrive with different delays atthe mobile terminal 122. The communication paths 130, 132 and 134 mayalso be differently attenuated. In the downlink, for example, thereceived signal at the mobile terminal 122 may be the sum of differentlyattenuated communication paths 130, 132 and/or 134 that may not besynchronized and that may dynamically change. Such a channel may bereferred to as a fading multipath channel. A fading multipath channelmay introduce interference but it may also introduce diversity anddegrees of freedom into the wireless channel. Communication systems withmultiple antennas at the base station and/or at the mobile terminal, forexample MIMO systems, may be particularly suited to exploit thecharacteristics of wireless channels and may extract large performancegains from a fading multipath channel that may result in significantlyincreased performance with respect to a communication system with asingle antenna at the base station 126 and at the mobile terminal 122,in particular for NLOS communication systems.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention. Referring toFIG. 1B, there is shown a MIMO transmitter 102 and a MIMO receiver 104,and antennas 106, 108, 110, 112, 114 and 116. There is also shown awireless channel comprising communication paths h₁₁, h₁₂, h₂₂, h₂₁,h_(2 NTX), h_(1 NTX), h_(NRX 1), h_(NRX 2), h_(NRX NTX), where h_(mn)may represent a channel coefficient from transmit antenna n to receiverantenna m. There may be N_(TX) transmitter antennas and N_(RX) receiverantennas. There is also shown transmit symbols x₁, x₂ and x_(NTX), andreceive symbols y₁, y₂ and y_(NRX).

The MIMO transmitter 102 may comprise suitable logic, circuitry and/orcode that may be enabled to generate transmit symbols x_(i) iε{1, 2, . .. N_(TX)} that may be transmitted by the transmit antennas, of which theantennas 106, 108 and 110 may be depicted in FIG. 1B. The MIMO receiver104 may comprise suitable logic, circuitry and/or code that may beenabled to process the receive symbols y_(i) iε{1, 2, . . . N_(RX)} thatmay be received by the receive antennas, of which the antennas 112, 114and 116 may be shown in FIG. 1B. An input-output relationship betweenthe transmitted and the received signal in a MIMO system may be writtenas:y=Hx+nwhere y=[y₁, y₂, . . . y_(NRX)]^(T) may be a column vector with N_(RX)elements, ^(T) may denote a vector transpose, H=[h_(ij)]: iε{1, 2, . . .N_(RX))}; jε{1, 2, . . . N_(TX)} may be a channel matrix of dimensionsN_(RX) by N_(TX), x=[x₁, x₂, . . . x_(NTX)]^(T) is a column vector withN_(TX) elements and n is a column vector of noise samples with N_(RX)elements. The channel matrix H may be written, for example, as H=UΣV^(H)using the Singular Value Decomposition (SVD), where .^(H) denotes theHermitian transpose, U is a N_(RX) by N_(TX) unitary matrix, Σ is aN_(TX) by N_(TX) diagonal matrix and V is N_(TX) by N_(TX) unitarymatrix. Other matrix decompositions that may diagonalize or transformthe matrix H may be used instead of the SVD. If the receiver algorithmimplemented in MIMO receiver 104 is, for example, an Ordered SuccessiveInterference Cancellation (OSIC), other matrix decompositions thatconvert the matrix H to lower/upper triangular may be appropriate. Onesuch decomposition may comprise Geometric Mean Decomposition (GMD),where H=QRP^(H), where R may be upper triangular with the geometric meanof the singular values of H on the diagonal elements, and Q and P may beunitary.

FIG. 2 is a block diagram illustrating an exemplary MIMO transceiverchain model, in accordance with an embodiment of the invention.Referring to FIG. 2, there is shown a MIMO system 200 comprising a MIMOtransmitter 202, a MIMO baseband equivalent channel 203, a MIMO receiver204, and an adder 208. The MIMO transmitter 202 may comprise atransmitter (TX) baseband processing block 210 and a transmit pre-codingblock 214. The MIMO baseband equivalent channel 203 may comprise awireless channel 206, a TX radio frequency (RF) processing block 212 anda receiver (RX) RF processing block 218. The MIMO receiver 204 maycomprise a pre-coding decoding block 216 and a RX baseband processingblock 220. There is also shown symbol vector s, pre-coded vector x,noise vector n, received vector y and channel-decoded vector y′.

The MIMO transmitter 202 may comprise a baseband processing block 210,which may comprise suitable logic, circuitry and/or code that may beenabled to generate a MIMO baseband transmit signal. The MIMO basebandtransmit signal may be communicated to a transmit pre-coding block 214.A baseband signal may be suitably coded for transmission over a wirelesschannel 206 in the transmit pre-coding block 214 that may comprisesuitable logic, circuitry and/or code that may enable it to performthese functions. The TX RF processing block 212 may comprise suitablelogic, circuitry and/or code that may enable a signal communicated tothe TX RF processing block 212 to be modulated to radio frequency (RF)for transmission over the wireless channel 206. The RX RF processingblock 218 may comprise suitable logic, circuitry and/or code that may beenabled to perform radio frequency front-end functionality to receivethe signal transmitted over the wireless channel 206. The RX RFprocessing block 218 may comprise suitable logic, circuitry and/or codethat may enable the demodulation of its input signals to baseband. Theadder 208 may depict the addition of noise to the received signal at theMIMO receiver 204. The MIMO receiver 204 may comprise the pre-codingdecoding block 216 that may linearly decode a received signal andcommunicate it to the RX baseband processing block 220. The RX basebandprocessing block 220 may comprise suitable logic, circuitry and/or logicthat may enable to apply further signal processing to baseband signal.

The MIMO transmitter 202 may comprise a baseband processing block 210,which may comprise suitable logic, circuitry and/or code that may beenabled to generate a MIMO baseband transmit signal. The MIMO basebandtransmit signal may be communicated to a transmit pre-coding block 214and may be the symbol vector s. The symbol vector s may be of dimensionN_(TX) by 1.

The transmit pre-coding block 214 may be enabled to apply a lineartransformation to the symbol vector s, so that x=Ws, where W may be ofdimension N_(TX) by length of s, and x=[x₁, x₂, . . . x_(NTX)]^(T). Eachelement of the pre-coded vector x may be transmitted on a differentantenna among N_(TX) available antennas.

The transmitted pre-coded vector x may traverse the MIMO basebandequivalent channel 203. From the N_(RX) receiver antennas, the receivedsignal y may be the signal x transformed by the MIMO baseband equivalentchannel 203 represented by a matrix H, plus a noise component given bythe noise vector n. As depicted by the adder 208, the received vector ymay be given by y=Hx+n=HWs+n. The received vector y may be communicatedto the pre-coding decoding block 216, where a linear decoding operationB may be applied to the received vector y to obtain the decoded vectory′=B^(H)y=B^(H)HWs+B^(H)n, where B may be a complex matrix ofappropriate dimensions. The decoded vector y′ may then be communicatedto the RX baseband processing block 220 where further signal processingmay be applied to the output of the pre-coding decoding block 216.

If the transfer function H of the MIMO baseband equivalent channel 203that may be applied to the transmitted pre-coded vector x is known bothat the MIMO transmitter 202 and the MIMO receiver 204, the channel maybe diagonalized by, for example, setting W=V and B=U, where H=UΣV^(H)may be the singular value decomposition. In these instances, the channeldecoded vector y′ may be given by the following relationship:y′=U ^(H) UΣV ^(H) Vs+U ^(H) n=Σs+U ^(H) nSince Σ may be a diagonal matrix, there may be no interference betweenthe elements of symbol vector s in y′ and hence the wirelesscommunications system may appear like a system with up to N_(TX)parallel single antenna wireless communication systems, for each elementof s, up to the rank of channel matrix H which may be less or equal toN_(TX). The operation of applying the matrix W to the vector s may bereferred to as precoding. The operation of making the wireless systemappear like a system of parallel non-interfering data streams due to theuse of multiple antennas, may lead to the use of the term spatial datastreams since each data stream may originate on different transmitantennas. The number of spatial data streams1≦N_(S)=r≦min{N_(TX),N_(RX)} that may be separated or decoupled may belimited by the rank r of the channel matrix H, as described above. Eachspatial stream originating at a transmit antenna may be modulated andcoded separately.

FIG. 3 is a block diagram of an exemplary MIMO with finite rate channelstate information feedback, in accordance with an embodiment of theinvention. Referring to FIG. 3, there is shown a MIMO system 300,comprising a partial MIMO transmitter 302, a partial MIMO receiver 304,a Wireless channel 306, an adder 308, and a feedback channel 320. Thepartial MIMO transmitter 302 may comprise a transmit pre-coding block314. The partial MIMO receiver 304 may comprise a pre-coding decodingblock 316, a channel estimation block 322, a channel quantization block310, a channel decomposition block 312, and a codebook processing block318. There is also shown a symbol vector s, a pre-coded vector x, anoise vector n, a received vector y, and a decoded vector y′.

The transmit pre-coding block 314, the wireless channel 306, the adder308 and the pre-coding decoding block 316 may be substantially similarto the transmit pre-coding block 214, the MIMO baseband equivalentchannel 203, the adder 208 and the pre-coding decoding block 216,illustrated in FIG. 2. The channel estimation block 322 may comprisesuitable logic, circuitry and/or logic to estimate the transfer functionof the wireless channel 206. The channel estimate may be communicated tothe channel decomposition block 312, which may comprise suitable logic,circuitry and/or code, which may be enabled to decompose the channel. Inthis regard, the decomposed channel may be communicated to the channelquantization block 310. The channel quantization block 310 may comprisesuitable logic, circuitry and/or code, which may be enabled to partlyquantize the channel onto a codebook. The codebook processing block 318may comprise suitable logic, circuitry and/or logic, which may beenabled to generate a codebook. The feedback channel 320 may represent achannel that may be enabled to carry channel state information from thepartial MIMO receiver 304 to the partial MIMO transmitter 302.

In many wireless systems, the channel state information, that is,knowledge of the channel transfer matrix H, may not be available at thetransmitter and the receiver. However, in order to utilize a pre-codingsystem as illustrated in FIG. 2, it may be desirable to have at leastpartial channel knowledge available at the transmitter. In the exemplaryembodiment of the invention disclosed in FIG. 2, the MIMO transmitter302 may require the unitary matrix V for pre-coding in the transmitpre-coding block 214 of MIMO transmitter 202.

In frequency division duplex (FDD) systems, the frequency band forcommunications from the base station to the mobile terminal, downlinkcommunications, may be different from the frequency band in the reversedirection, uplink communications. Because of a difference in frequencybands, a channel measurement in the uplink may not generally be usefulfor the downlink and vice versa. In these instances, the measurementsmay only be made at the receiver and channel state information (CSI) maybe communicated back to the transmitter via feedback. For this reason,the CSI may be fed back to the transmit pre-coding block 314 of thepartial MIMO transmitter 302 from the partial MIMO receiver 304 via thefeedback channel 320. The transmit pre-coding block 314, the wirelesschannel 306, and the adder 308 are substantially similar to thecorresponding blocks 214, 203 and 208, illustrated in FIG. 2.

At the partial MIMO receiver 304, the received signal y may be used toestimate the channel transfer function H by Ĥ in the channel estimationblock 322. The estimate may further be decomposed into, for example, adiagonal or triangular form, depending on a particular receiverimplementation, as explained for FIG. 2. For example, the channeldecomposition block 312 may perform an SVD:

=

{circumflex over (Σ)}

^(H). In the case of a plurality of antennas, the dimensions of thematrices U, Σ and V may grow quickly. In these instances, it may bedesirable to quantize the matrix

into a matrix Vq of dimensions NTX by NTX, where Vq may be chosen frompre-defined finite set of unitary matrices C={Vi}. The set of unitarymatrices C may be referred to as the codebook. By finding a matrix Vqfrom the codebook that may be, in some sense, closest to the matrix

, it may suffice to transmit the index q to the transmit pre-codingblock 314 via the feedback channel 320 from the channel quantizationblock 310, if the partial MIMO transmitter 302 may know the codebook C.The codebook C may be varying much slower than the channel transferfunction H and it may suffice to periodically update the codebook C inthe transmit pre-coding block 314 from the codebook processing block 318via the feedback channel 320. The codebook C may be chosen to be staticor adaptive. Furthermore, the codebook C may also be chosen, adaptivelyor non-adaptively, from a set of codebooks, which may compriseadaptively and/or statically designed codebooks. In these instances, thepartial MIMO receiver 304 may inform the partial MIMO transmitter 302 ofthe codebook in use at any given instant in time. Hence, the channel Hmay be estimated in the channel estimation block 322 and decomposed inthe channel decomposition block 312.

In the channel quantization block 310, a matrix, for example {circumflexover (V)} may be quantized into a matrix V_(q) and the index q may befed back to the partial MIMO transmitter 302 via the feedback channel320. The codebook C may also be chosen time invariant. Furthermore, thecodebook C may also be chosen, adaptively or non-adaptively, from a setof codebooks, which may comprise adaptively and/or statically designedcodebooks, as described above. Less frequently than the index q, thecodebook C from the codebook processing block 318 may be transmitted tothe partial MIMO transmitter 302 via the feedback channel 320. Tofeedback the index q, M bits may suffice when the cardinality |C| of thecodebook C may be less or equal to |C|≦2^(M).

The transmit pre-coding block 314 may perform, for example, the lineartransformation x=V_(q)s. The pre-coding decoding block 316 at thereceiver may implement the linear transformation y′=Û^(H)y. In someinstances, the rank r of the channel matrix H may be less than thenumber of transmit antennas r≦N_(TX). In these instances, it may bedesirable to map a reduced number of spatial streams into the vector x,as described for FIG. 2. For example, the vector s may be chosen, sothat x=Ws, where W may be of dimension N_(TX) by the length of s and thelength of s may be the number of spatial streams, generally smaller thanthe rank r. The matrix W may be constructed, for example, from adesirable choice of columns from V_(q). In another embodiment of theinvention, the vector x may be generated from x=V_(q)s, as describedabove, and some suitably chosen elements of the vector s of lengthN_(TX) may be set to zero, so that generally the non-zero elements inthe vector s may be less than the rank r. In these instances, theelements in s that may be set to zero may correspond to non-utilizedspatial streams. The feedback of the index q, and associatedinformation, may be referred to as Pre-Coding Matrix Index (PMI)information.

In some instances, it may be possible that the different spatial streamsmay experience significantly different channel conditions. For example,an attenuation coefficient of one spatial stream may be significantlydifferent from an attenuation coefficient of another spatial stream. Forexample, the Signal-to-Noise Ratio (SNR) or another performance measuremay differ between the spatial streams. Accordingly, the modulationand/or coding of each spatial stream may be adapted independently.Adapting the modulation format and the coding rate for each spatialstream (by adapting the transmitted symbols, for example) may be enabledby feeding back channel state information and/or channel-basedinformation from the MIMO receiver 304 to the MIMO transmitter 302 viathe feedback channel 320. Feedback information that may be utilized todetermine suitable modulation and coding protocols for the transmit datamay be referred to as Channel Quality Indicator (CQI) information. Inaccordance with various embodiments of the invention, the CQIinformation may be, for example, a Signal-to-Noise-and-InterferenceRatio (SINR) that may be mapped to a suitable modulation and codingconfiguration. In another embodiment of the invention, the MIMO receiver304 may directly feedback a desirable modulation and codingconfiguration, based on estimated channel quality, for example.

The modulation and coding for each spatial stream may be chosen from amodulation coding set (MCS), which may comprise combinations ofmodulation constellations and coding rates that may be employed by thepartial MIMO transmitter 302. For example, the modulation may be chosenfrom, but is not limited to, QPSK, 16QAM or 64QAM, where QPSK may denotequadrature phase shift keying and K-QAM may denote quadrature amplitudemodulation with K constellation points. A coding rate may be chosen tobe, for example, ⅓, ⅕ or ¾, whereby any rational number smaller than 1may be feasible. A modulation coding set may comprise elements that maybe formed by combining a modulation type with a coding rate. Anexemplary element of a modulation coding set may be ‘QPSK ⅓’, which maydenote a QPSK modulation with a coding rate of ⅓. An MCS may comprise Nelements. In this case, the MCS may be referred to as an N-level MCS. Inorder to select an element from an N-level MCS at the partial MIMOreceiver 304 and feed back the index indicating the appropriate elementin the MCS from the partial MIMO receiver 304 to the partial MIMOtransmitter 302 via the feedback channel 320, B≧log₂(N) bits of feedbackmay be required per spatial stream.

In order to reduce the number of bits required for feedback, adifferential scheme may be implemented. In these instances, B≧log₂(N)bits may be transmitted for the spatial stream 1, for example, totransmit an index to an element of the MCS, as described above. Theparameter s_(k) may denote an MCS feedback value for spatial stream k.For the spatial streams 2 though N_(S), an index offset value s_(k) maybe fed back from the partial MIMO receiver 304 to the partial MIMOtransmitter 302. Such an offset value may, for example, take the valuess_(k)ε{0, ±1, ±2, ±3}: k=2, . . . , N_(S). In this exemplary case, forspatial stream 2 through N_(S), B_(d)=3 bits of feedback may besufficient to feed back an offset value s_(k): k≠1. The required indexto the MCS for spatial stream k may then be obtained from the feedbackvalue for spatial stream 1 and the offset s_(k). The index q(k) maydenote the index of the desired element in the MCS for user k. Hence,applying the above procedure, the partial MIMO transmitter 302 maydetermine the indices q(k) according to the following relationship:q(j)=s _(j):requiring B≧log 2(N) feedback bitsq(k)=q(j)+s _(k) :∀k≠j,B _(d) feedback bits required for s _(k)  (1)where j=1 may be as chosen above. The index j may be chosen tocorrespond to an arbitrary spatial stream, such that jε{1, 2, . . . ,N_(S)}. For ease of exposition and clarity, j=1 may be assumed in thefollowing description. When B_(d)<B, the number of bits that may be fedback from the partial MIMO receiver 304 to the partial MIMO transmitter302 may be reduced. In some instances, due to a reduction in the numberof feedback bits, the range of indices q(k) that may be addressed byq(k): k≠j may be limited to a subset of the MCS, since the addressableelements in the MCS and their associated indices q(k) may depend on thevalue s_(j)=s₁.

A further reduction in the number of feedback bits may be achieved byusing a differential scheme also for spatial stream j=1. This may bedone in instances where the channel conditions vary slowly enough toenable differential tracking of the new index based on an offset valueadded to the last instance of the index value. In this case, the indexat time n for user k may be defined by the following relationship:q ₀(j)=s _(j):requiring B≧log 2(N) feedback bitsq _(n)(j)=q _(n-1)(j)+s _(j) :B _(d) feedback bits required  (2)q _(n)(k)=q _(n)(j)+s _(k) :∀k≠j,B _(d) feedback bits requiredIn this case, hence, the initial index for the spatial stream j=1 may befed back using B bits, which may address any element in the MCS. Forsubsequent indices the channel may change slowly enough so that theprevious index q_(n-1)(j) may be used to determine the new indexq_(n)(j). It may be desirable to reinitialize q_(n)(j) occasionally.

It may be desirable to choose an appropriate number of levels for theMCS. In principle, N, the number of elements or levels of an N-levelMCS, may be chosen to be any positive integer. However, since the indexto an element of an N-level MCS may be fed back from the partial MIMOreceiver 304 to the partial MIMO transmitter 302, it may be efficient tochoose N as a power of 2. Furthermore, it may be undesirable to use bothmany and few levels. With few levels, the MCS may be relatively coarse,which may lead to a selection of a level that may be inefficient for thegiven channel conditions. On the other hand, an MCS with a large numberof levels may provide a highly efficient match between the channelconditions and the selected level in the MCS. It may take comparativelylong until the system settles, that is, the transient phase, alsoreferred to as settling time, may be long. In addition, with a largenumber of levels, the differential protocol for the spatial streams 2through N_(S) introduced above, may potentially result in a smalldynamic range, which may be undesirable.

FIG. 4 is a time-frequency diagram illustrating a time-frequencywireless channel, in accordance with an embodiment of the invention.Referring to FIG. 4, there is shown a time-frequency diagram 400,comprising a detail blow-up 402. The frequency axis may be divided intoseveral sub-divisions. A sub-band may be given by a bandwidth f_(SB). Asub-band may comprise, for example, one or more resource blocks ofbandwidth f_(RB). A resource block may comprise a bandwidth comprisingone or more carrier spacings of bandwidth f_(d). In an OrthogonalFrequency Division Multiplexing (OFDM) system, for example, the carrierspacing may be given by the bandwidth of a tone and/or between tones. ACQI reporting unit may comprise a bandwidth f_(CQI). A PMI reportingunit may comprise a bandwidth f_(PMI).

The time axis may be sub-divided similarly to the frequency axis. Thereis shown a t_(CQI), a t_(PMI) and a t_(s) subdivision. t_(CQI) andt_(PMI) may be reporting intervals between CQI and PMI feedbackmessages, respectively. The t_(s) subdivision may be a more fundamentaltime unit, for example, a channel sampling time. Referring to FIG. 4,there is also shown a CQI time-frequency reporting unit, marked by ahatched pattern, and a PMI reporting unit, marked by a cross-hatchedpattern.

In the description of the wireless channel for FIG. 2, the wirelesschannel for a MIMO system may be described by a channel matrix H.However, the matrix H may represent the channel between transmit andreceive antennas by a scalar, as may be see from the channel modeldescribing the matrix H: H=[hij]: iε{1, 2, . . . N_(RX)}, jε{1, 2, . . .N_(TX)}. The wireless channel, in general, may be a function of time andfrequency and may be approximately constant only over a small area ofthe time-frequency plane. This area may be determined by the channelconditions, for example the channel coherence bandwidth and the channelcoherence time. These variables may be determined by a variety ofenvironmental factors, for example, Doppler spread due to movements.Hence, the channel matrix may be function of both time and frequency andmay be written more accurately as H(f,t)=[hij(f,t)]: iε{1, 2, . . .N_(RX)}, jε{1, 2, . . . N_(TX)}. For notational illustrative purposes,the time and frequency dependency is not shown, however. With regard toFIG. 4, in some instances a matrix H may be measured for eachtime-frequency slice of area t_(s) by f_(d), as illustrated in thedetail blow-up 402.

In an OFDM system, such a channel matrix H may correspond to a channelestimate of a single OFDM sub-carrier, also referred to as a tone, in asampling interval of length t_(s). Since a channel measurement H may bemade for each unit of area t_(s) by f_(d) of the wireless time-frequencychannel, a large amount of channel data may be available. However, sincefeedback capacity from the MIMO receiver 304 to the MIMO transmitter 302via the feedback channel 320 may be limited, it may be necessary toreduce the resolution of the channel feedback messages and reportchannel measurements that may be a function of a number of channelmeasurements. Similarly, in order to reduce the rate at which feedbackmessages may be sent back to the MIMO transmitter 302, the feedbackmessages may be sent every multiple of, for example, the sampling timet_(s). The resolution may be reduced, for example, by averaging themeasured channel matrices H in time and frequency over the reportingarea in the time-frequency plane, as illustrated for a CQI reportingunit and/or a PMI reporting unit in FIG. 4.

In one exemplary embodiment of the invention illustrated in FIG. 4, aCQI reporting block may comprise a bandwidth of f_(CQI), which maycomprise, for example, a plurality of resource blocks of bandwidthf_(RB). As shown in the detail blow-up 402, the Resource block maycomprise, for example 8 carrier spacing blocks of bandwidth f_(d) each.In the time dimension, the CQI reporting unit may comprise a timeinterval of t_(CQI), which may, for example, comprise 3 sampling periodst_(s). Hence, as illustrated in FIG. 4, the available bandwidth may bedivided into 5 sub-bands and a CQI feedback message may be generated foreach sub-band, every period t_(CQI), for example. The CQI reporting unitmay comprise a feedback value that may be a function of the channelmeasurements obtained for the time-frequency slice covered by the CQIreporting unit. For example, an average SINR over the time and frequencyslice of the CQI reporting unit may be computed to determine anappropriate average coding and modulation level to be fed back. Hence,to reduce feedback requirements, a plurality of channel measurements Hmay be processed into one feedback message per PMI/CQI reporting unit.In some instances, a plurality of feedback messages may be desirable.For example, a CQI feedback message may comprise one determined SINRvalue for one CQI reporting unit.

Similarly, a suitable measure for feedback may be determined for the PMIfeedback message. For example, an average matrix H′ may be computed fromthe measured matrices H covered by the time-frequency slide of the PMIreporting unit. In another embodiment of the invention, the measuredmatrix H for a centrally placed time-frequency slice may be fed back,averaged over time, for example.

The CQI reporting unit and the PMI reporting unit may be arbitrarilysized, in accordance with an embodiment of the invention. In addition,the CQI/PMI reporting unit dimensions may be adjusted dynamically, forexample as a function of the available feedback capacity and/or thechannel conditions. Similarly, the dimensions of the sub-divisions intime and/or frequency may be chosen arbitrarily. For example, in oneembodiment of the invention, a resource block may comprise 12 OFDMtones. The generation of the CQI/PMI messages may not be limited toaverages and may be any arbitrary function that may be at least afunction of channel conditions and/or channel measurements.

In another embodiment of the invention, the PMI and/or CQI reportingunits may be variably sized in time and/or frequency. In other words, aCQI/PMI reporting unit may be of different size, depending on theabsolute frequency and the feedback frequency may be dependent on thefrequency. For example, an entire channel may be 5 MHz wide. Inaccordance with an embodiment of the invention, a first CQI/PMI maycover the bandwidth from 0-1 MHz and may feedback a message every xseconds. A second CQI/PMI may cover the bandwidth from 1-4.5 MHz and mayfeedback a message every 3x seconds. A third CQI/PMI may cover thebandwidth from 4.5-5 MHz and may feedback a message every x/2 seconds.In some instances, neighboring PMI/CQI reporting units may also overlapin time and/or frequency.

FIG. 5 is a flow chart illustrating exemplary adjustment of CQI and PMIreporting units, in accordance with an embodiment of the invention. Asdescribed for FIG. 4, the CQI/PMI reporting units may be adjusted in thetime and frequency domain to represent a variable size time-frequencyslice of the wireless channel. In addition, the CQI/PMI reporting unitfunctions that may be used to process channel measurements to generateCQI/PMI feedback messages may be adjustable, in accordance with variousembodiments of the invention.

In one embodiment of the invention, the CQI/PMI reporting units may beadjusted dynamically, based on channel conditions, for example, asillustrated in FIG. 5. In step 504, channel measurements may be obtainedas a basis for estimating desirable parameters. Channel measurements maybe obtained, for example, by transmitting training signals from a MIMOtransmitter 302 to a MIMO receiver 304. The training signals may beenabled to permit estimation of the MIMO channel 320, for example, atthe MIMO receiver 304. For example, the training signals may possessgood autocorrelation properties and may often be chosen orthogonalbetween different transmit antennas in one or more dimension, forexample in the time domain, frequency domain or code domain. In somecases, semi-blind or blind channel estimation may be utilized, wherebyno explicit training signals may be required and certain signalcharacteristics may be exploited to allow channel estimation, forexample finite-alphabet, constant modulus and/or cyclo-stationarity ofthe transmit signals.

In step 506, the channel measurements may be processed to estimate oneor more parameters that may be desirable for the adjustment of theCQI/PMI reporting units. Such parameters may be channel characteristics.For example, in the case of macrocells, relevant parameters may comprisethe K-factor, delay spread, Doppler spread and/or angular spread of thechannel. The K-factor is a factor that may indicate the channel powerspread and may be an indicator about the strength of line-of-sightmultipath components. A high K-factor may indicate a significant LOScomponent. The K-factor may typically decrease exponentially with theseparation distance between the base station (also referred to as Node Bin UMTS) and the mobile terminal (also referred to as User Equipment,UE, in UMTS). The delay spread may be a parameter related to thedistribution of received multipath components at the MIMO receiver 304and may increase with increasing separation distance between the mobileterminal and the base station. In addition, the delay spread may bedependent on the terrain, with hilly terrain generally resulting inhigher delay spread. The coherence bandwidth may be derived from thedelay spread and may provide an indication about how variable thewireless channel may be in the frequency domain. Hence, delay spread andcoherence bandwidth may be parameters that may help to decide thebandwidth of the reporting unit, for example.

Doppler spread may be another desirable parameter. The Doppler spreadmay be largely invariant with range but may depend on the speed at whichthe mobile terminal may travel, for example. Doppler spread may berelated to coherence time and may hence be a desirable parameter todetermine the feedback frequency to accommodate time-variability of thechannel. Another desirable parameter may be angular spread, which mayindicate the range of directions from which significant signalcomponents may be received. This information may be desirable forbeamforming and pre-coding matrix purposes and may help exploit antennadiversity.

Based on some parameters, comprising, for example, the above listedparameters, the CQI/PMI reporting unit may be adjusted in both time andfrequency in step 506. In step 510, the adjustment loop may bereinitialized and/or terminated. In some instances, the CQI and the PMIfeedback resources may be allocated independently. In another embodimentof the invention, the CQI and PMI feedback resources may be allocatedjointly, in view of constraints. For example, in some instances, amaximum number of bits may be available for feedback and it may bedetermined how to divide available resources between PMI and CQIfeedback.

In accordance with an embodiment of the invention, a method and systemfor adaptive allocation of feedback resources for CQI and transmitpre-coding may comprise assigning a bandwidth and a feedback period toone or more CQI reporting units and a bandwidth and a feedback period toone or more PMI reporting units, as illustrated in FIG. 4. One or morefeedback messages may be generated based at least on channel stateinformation associated with the bandwidth and the feedback periodassigned to the one or more CQI reporting units and with the bandwidthand the feedback period assigned to the one or more PMI reporting units,as described for FIG. 4.

As described in FIG. 4 and FIG. 5, the bandwidth and the feedback periodof the CQI reporting units and/or the PMI reporting units may beadjusted dynamically and/or adaptively. Based on the channel stateinformation, as described for FIG. 5, for example, or as a function of afeedback rate, the bandwidth and/or the feedback period may be assigned.The associated channel state information may comprise channelmeasurements, for example channel matrix H that may be associated withthe assigned bandwidth and the assigned feedback period. Processing thechannel state information may be achieved by averaging the channelmeasurements, for example matrices H or SINRs of OFDM tones as describedfor FIG. 4, or by an arbitrary function of the channel measurements. Theone or more feedback messages for the one or more CQI reporting unitsmay comprise modulation and coding levels orSignal-Noise-and-Interference ratios. The one or more feedback messagesfor the one or more PMI reporting units may comprise one or more indicesinto a codebook or one or more matrices.

Another embodiment of the invention may provide a machine-readablestorage, having stored thereon, a computer program having at least onecode section executable by a machine, thereby causing the machine toperform the steps as described above for a method and system foradaptive allocation of feedback resources for CQI and transmitpre-coding.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

1. A method for processing communication signals, the method comprising:assigning a bandwidth and a feedback period to one or more CQI reportingunits and a bandwidth and a feedback period to one or more PMI reportingunits; and generating one or more feedback messages based at least onuplink channel state information associated with said bandwidth and saidfeedback period assigned to said one or more CQI reporting units andwith said bandwidth and said feedback period assigned to said one ormore PMI reporting units.
 2. The method according to claim 1, comprisingdynamically adjusting said bandwidth and said feedback period of saidCQI reporting units and/or said PMI reporting units.
 3. The methodaccording to claim 1, comprising adaptively adjusting said bandwidth andsaid feedback period of said CQI reporting units and/or said PMIreporting units.
 4. The method according to claim 1, comprisingassigning said bandwidth and/or said feedback period based on uplinkchannel state information.
 5. The method according to claim 1,comprising assigning said bandwidth and/or said feedback period as afunction of a feedback rate.
 6. The method according to claim 1, whereinsaid associated uplink channel state information comprises channelmeasurements associated with said assigned bandwidth and said assignedfeedback period.
 7. The method according to claim 6, comprisingprocessing said uplink channel state information by averaging saidchannel measurements.
 8. The method according to claim 6, comprisingprocessing said uplink channel state information by an arbitraryfunction of said channel measurements.
 9. The method according to claim1, wherein said one or more feedback messages for said one or more CQIreporting units comprise modulation and coding levels.
 10. The methodaccording to claim 1, wherein said one or more feedback messages forsaid one or more CQI reporting units compriseSignal-to-Noise-and-Interference Ratios (SINR).
 11. The method accordingto claim 1, wherein said one or more feedback messages for said one ormore PMI reporting units comprise one or more indices into a codebook.12. The method according to claim 1, wherein said one or more feedbackmessages for said one or more PMI reporting units comprise one or morematrices.
 13. A system for processing communication signals, the systemcomprising: one or more circuits, said one or more circuits enable:assignment of a bandwidth and of a feedback period to one or more CQIreporting units and a bandwidth and a feedback period to one or more PMIreporting units; and generation of one or more feedback messages basedat least on uplink channel state information associated with saidbandwidth and said feedback period assigned to said one or more CQIreporting units and with said bandwidth and said feedback periodassigned to said one or more PMI reporting units.
 14. The systemaccording to claim 13, wherein said one or more circuits dynamicallyadjust said bandwidth and said feedback period of said CQI reportingunits and/or said PMI reporting units.
 15. The system according to claim13, wherein said one or more circuits adaptively adjust said bandwidthand said feedback period of said CQI reporting units and/or said PMIreporting units.
 16. The system according to claim 13, wherein said oneor more circuits assign said bandwidth and/or said feedback period basedon uplink channel state information.
 17. The system according to claim13, wherein said one or more circuits assign said bandwidth and/or saidfeedback period as a function of a feedback rate.
 18. The systemaccording to claim 13, wherein said associated uplink channel stateinformation comprises channel measurements associated with said assignedbandwidth and said assigned feedback period.
 19. The system according toclaim 18, wherein said one or more circuits process said uplink channelstate information by averaging said channel measurements.
 20. The systemaccording to claim 18, wherein said one or more circuits process saiduplink channel state information by an arbitrary function of saidchannel measurements.
 21. The system according to claim 13, wherein saidone or more feedback messages for said one or more CQI reporting unitscomprise modulation and coding levels.
 22. The system according to claim13, wherein said one or more feedback messages for said one or more CQIreporting units comprise Signal-to-Noise-and-Interference Ratios (SINR).23. The system according to claim 13, wherein said one or more feedbackmessages for said one or more PMI reporting units comprise one or moreindices into a codebook.
 24. The system according to claim 13, whereinsaid one or more feedback messages for said one or more PMI reportingunits comprise one or more matrices.